The present invention relates to a wear-resistant alumina coated tool for use in cutting tools, etc., which is excellent in a resistance to coat-flaking, and also relates to a production method thereof.
Generally, a coated tool is produced by forming a hard coating on the surface of a substrate body made of a cemented carbide, a high speed steel, a special steel, etc. by the chemical vapor deposition or the physical vapor deposition. The coated tool has found wide practical applications because of having both the wear resistance resulting from the hard coating and the toughness resulting from the substrate body. Since the cutting edge of the cutting tools reaches about 1000.degree. C. particularly during the machining of a highly hard material at a high cutting speed, and the cutting tools subjected to wear due to contacting with the workpiece materials and an intermittent mechanical shock, the coated tools having both the wear resistance and the toughness have been found useful.
The hard coating is a single layer or multilayer structure of a non-oxide coating having a high wear resistance and toughness, or an oxide coating having a good oxidation resistance. The non-oxide coating may be carbide, nitride, carbonitride of a metal selected from the Groups IVa, Va and VIa of the Periodic Table such as TiN, TiC and TiCN, and an oxide coating may be .alpha.-Al.sub.2 O.sub.3, .kappa.-Al.sub.2 O.sub.3, etc.
The main disadvantage of the non-oxide coating comprising the carbide, nitride, carbonitride, etc., is its particular sensitivity to oxidation. To minimize the oxidation of the non-oxide coating layer, there has been proposed a multi-layered structure in which an oxide coating such as alumina having a good oxidation resistance is coated on the non-oxide coating. However, the non-oxide/oxide multi-layered structure is low in the bonding strength between the non-oxide coating and the oxide coating and unstable in the mechanical strength at a higher temperature.
Although, .kappa.-Al.sub.2 O.sub.3 layer is tightly bonded to the non-oxide layer and formed at relatively low temperature of 1000 to 1020.degree. C., the cracking in the coating layer and the flaking or removal of the .kappa.-Al.sub.2 O.sub.3 coating layer from the coated body frequently occur during the machining operation at a high temperature because of the volume change caused by the phase transfer from the metastable .kappa.-Al.sub.2 O.sub.3 to the stable .alpha.-Al.sub.2 O.sub.3.
On the other hand, .alpha.-Al.sub.2 O.sub.3 is stable at a high temperature and has a good oxidation resistance and high temperature property. However, the formation of .alpha.-Al.sub.2 O.sub.3 coating directly on the non-oxide layer requires a high temperature of about 1040.degree. C. or higher which makes the .alpha.-Al.sub.2 O.sub.3 grains coarser with a grain size of 1 to 6 .mu.m, this resulting in micropores and crystallographic defects to reduce the mechanical strength of the alumina coating. Also, when exposed to a high temperature, the property of the non-oxide layer is likely to be deteriorated, for example, due to the diffusion of W and/or Co, etc.
To eliminate the problem of the high formation temperature, the conventional technique has proposed that the surface of the non-oxide coating is first oxidized to form an oxidized layer and then the .alpha.-Al.sub.2 O.sub.3 coating is formed on the oxidized layer at a relatively low temperature of 1000 to 1020.degree. C. However, the .alpha.-Al.sub.2 O.sub.3 coating formed by such a method is insufficient in the bonding strength with the non-oxide layer, and still suffers from the fast flaking or removal of the .alpha.-Al.sub.2 O.sub.3 coating from the non-oxide under coating during the cutting operation.
To improve the bonding strength between the oxide coating mainly comprising .alpha.-Al.sub.2 O.sub.3 and the non-oxide under coating, several methods have been proposed. Japanese Patent Laid-Open No. 63-195268 has proposed a method in which TiCNO coating of 0.5 .mu.m thick is first formed at 1100.degree. C. or 1030.degree. C. and then the alumina layer is formed thereon at 960.degree. C. or 1000.degree. C. Japanese Patent Laid-Open No. 2-30406 has proposed to form the alumina layer on TiCO or TiCNO coating of 1 .mu.m thick. Japanese Patent Laid-Open No. 5-345976 has proposed to form the alumina layer at 1000.degree. C. on TiCNO or TiCO coating of 0.5 or 3 .mu.m thick having a grain size of 0.2 to 1.5 .mu.m formed at 1000.degree. C. by using TiCl.sub.4, CO, H.sub.2, and N.sub.2 gas. U.S. Pat. No. 5,487,625 has disclosed a method in which a substrate body is coated with a layer of TiCNO and then the layer of .alpha.-Al.sub.2 O.sub.3 is deposited thereon by carefully controlling the oxidation potential of the hydrogen carrier gas below 20 ppm of H.sub.2 O prior to the nucleation of Al.sub.2 O.sub.3. The nucleation of Al.sub.2 O.sub.3 is started by sequentially adding the reacting gases in the order of CO.sub.2, CO and AlCl.sub.3 while keeping the temperature at about 1000.degree. C. during the nucleation.
However, the bonding strength between the alumina layer and the non-oxide under coating is still insufficient in the coating structure obtained by the above proposed methods.